Diversity receiver



1.... HO 0\ l `Oct. 27, 1936.

G. L aEERs DIVERS ITY RECEIVER 2 sheets-sheet '1 Filed Oct. 3l.. 1933UNITED STATES PATENT OFFICE nrvaasrrr RECEIVER George L. Beers,Collingswood, N. J., assignor to Radio Corporation of America, acorporation of Delaware Application october 31, 193s, serial No.assesso- 26 claims (ci. 25o-zo) -My invention relates to radio receivingsystems. and particularly to radio receiving systems of the diversityreceiver type employing a plurality of antennas. It is well known thatduring periods of fading,

It is a still further object of my invention to provide an improvedultra-high frequency receiver system for reducing the effect ofinterference patterns near the receiver.

In one embodiment of my invention, a diversity the variations inamplitude of the signals derived radio receiving system is provided witha plurality 5 from antennas spaced apart geographically do of antennasand with switching means for sucnot occur simultaneously. Advantage hasbeen cessively connecting the antennas to the input taken of this factby employing several receiving circuit of a single receiver. In theevent that a l antennas spaced apart several wave lengths or region oflow signal strength is produced at an 10 more, each antenna beingconnected to a radio antenna connected to the receiver another isreceiver tuned to the frequency of the desired automatically selectedand connected with the signal. By coupling the output circuits of thereceiver. The antennas may be spaced apart a receivers to a commontranslating device, such distance, such as several wave lengths,sufficient l asan audio frequency amplifier, a signal may to make itunlikely that severe fading will occur 15 be obtained which does notfade as much as at all the antennas at the same time, or they whenemploying conventional receiving apparamay be spaced apart solely forthe purpose of tus. A diversity receiving system of this type isreducing the effects otinterference" patterns. described in the Proc. I.R. E. for April, 1931, The antenna switching means includes a 2o v01,19, page 531, switching mechanism operated by means of a re- 20 `I havediscovered that when signaling by means lay so connected to the receiverthat as long as a of radio waves having a very short wave length signalis of normal strength, the relay does not (less than ten meters, forexample) the use of a operate. As soon as a signal fades, however, orplurality of geographically spaced antennas is if the receiver is tunedinto a diiferent transmitdesirable to overcome the effect of aphenomenon ting Station having aregion of minimum Signal 25 distinctfrom the above mentioned fading. The Strength at the antenna in use, therelay iS Opelphenomenon referred to is that of the setting up ated toactuate the above-mentioned switching of interference patterns by signalwaves reflected mechanism and a different antenna is connected frompoints near the receiver. Thus, for the tothe receiver input circuit. Ifthe signal is signal from a certain transmitter, there may be fading 0rWeak for any reason at this antenna 30 l points of minimum and maximumsignal strength.f also, the relay will again operate and a stili dif- Ifa single antenna were employed, it might be ferent antenna will beconnected to the receiver located at one of the points of minimum signalinput Circuit. strength, Other features and advantages of my inven- .35Since the points of minimum and maximumtion Will .appear from thefollowing description signal strength will not coincide for differenttaken in Connectin With the accompanying drawtransmitters, an antenna solocated as to receive ings, in Which efllciently from one transmittermay pick up a Fig. 1 is a circuit diagram of one embodiment very weaksignal from another transmitter. A of my invention; and

.40 receiver of the diversity type has, therefore, been Fig. 2 is acircuit diagram of a modified form 40 found desirable for the receptionof ultra-high of input circuit for the apparatus shown in frequencysignals. Fig. l. e

Heretofore, diversity receiving systems gener- Referring to Fig. l, myinvention is shown ally have had the disadvantage that a separateapplied to a superheterodyne receiver comprising receiver unit wasrequired for each antenna, a radio frequency amplifier l, Which isprovided 45 thereby increasing the cost of such systems and with aninput circuit coupled to an antenna ,A making it more diincult to tunein a signal. Q switch 3 through a suitable tuned radio frequency Anobject of my invention is to provide ani transformer 5. improveddiversity receiver system in which only In the example illustra-ted, anyOne 0f the i one radio receiver is required. l' three antennas l, S, andli, may be connected 5o It is a further object. of my invention toprovide to the input circuit of the amplifier I by rotating an improvedapparatus for reducing the effect of a switch arm I3. In the diagram,the antenna 9 "selective fading, that is, the signal distortion is shownconnected to the receiver. the connecresulting from the independentfading of side tion being from a switch segment l5 through the bands andcarrier wave of a modulated signal. switch arm i3, to an opposite switchsegment l1, 55

The radio frequency amplifier I is suitablyv coupled to the inputcircuit of a first detector 21 through a tuned radio frequencytransformer 29.

An oscillator 3I is coupled to the input circuit of the detector 21 forconverting the incoming radio frequency signal to an intermediatefrequency signal in the usual manner.

The detector 21 is, in turn, coupled to an intermediate frequencyamplifier 33 through a transformer 35 which is tuned to the intermediatefrequency.

The output of the amplier 33 is fed through a tuned intermediatefrequency transformer 31 to a diode rectifier 39 which functions as thesecond detector of the receiver. The rectifier circuit includes aresistor 4I connected in series with the secondary of the transformer31, this resistor being shunted by an intermediate frequency bypasscondenser 43. 'Ihe anode end of the resistor 4I is connected to ground.

The audio signal output of the second detector 39 appearing across theresistor 4I is impressed upon the input circuit of an audio frequencyamplifier by means of a suitable coupling condenser 41 and grid resistor48. The audio frequency amplifier 45 is provided with an output circuitwhich may be coupled to any suitable amplifying or translating device(not shown), by means of an audio frequency transformer 49.

The screen grids and plates of the radio frequency amplifier I, the rstdetector 21, and the intermediate frequency amplifier 33 are suppliedwith operating potentials from any suitable source such as tap points onthe usual voltage divider resistor 5I connected to a power supply means53.

In accordance with my invention, an electric discharge device or tube55, which may be an ordinary three element or four element tube, isprovided for controlling a second electric discharge device such as agas tube relay 51 which functions to control the ratchet 2I and theantenna switch 3.

The control grid 59 of the tube 55 is connected through a conductor 6Uto that end of the resistor 4I in the detector circuit which is positivewhen current is iiowing through it. The cathode 6I of the tube 55 isconnected to the voltage divider 5I at a point which is positive withrespect to ground.

If a screen grid tube is utilized, as illustrated, the screen grid 58may be supplied with an operating potential from any suitable sourcesuch as a battery 62 or the voltage divider 5I.

The input circuit of the tube may be traced from the control electrode59 through the resistor 4I in the second detector circuit through groundto the voltage divider 5I, and through a lower section 63 of the voltagedivider, through a conductor 65 to the cathode 6I of the tube 55.

It will be apparent that, if there is no incoming signal, there will beno voltage drop along resistor 4I, and the control grid 59 will be at anegative potential determined by the voltage drop in the resistorsection 63. An incoming signal will decrease the negative bias on thegrid 59 since the voltage drop produced in the resistor 4I will be inopposition to the voltage drop in the resistor section 63.

Alternating current is supplied to the plate circuit of the tube 55through a transformer 61.

This alternating current is rectified by the tube 55, when the negativepotential on the grid 59 is reduced sufficiently, and is supplied to avolume control resistor 69 which is connected in the plate circuit oftube 55. The plate circuit of tube 55 may be traced from the plate 1Ithrough the secondary of the transformer 61, through the volume controlresistor 69 to ground, through ground to the voltage divider 5I, througha lower section 63 of the voltage divider, and through the conductor 65to the cathode 6I.

The volume control resistor 69 is shunted by the usual filter condenser13 for preventing audio frequency signals from being applied to thecontrol grids oi the tubes I, 21 and 33, these grids being connected tothe volume control resistor so that they are automatically biased inaccordance with the strength of an incoming signal.

The above-described volume control circuit, which has as one of itsfeatures the app1ica tion of alternating voltage to the plate of avolume control tube, is described and claimed in my copendingapplication Serial No. 703,034, led December 19, 1933, and assigned tothe same assignee as this application.

As stated above, when there is no incoming signal, the control grid 59of the vacuum tube 55 is negative with respect to the cathode 5I due tothe biasing potential on the voltage divider section 63, since there isno opposing voltage drop along the detector resistor 4I. This negativevoltage on the grid 59 is made suilicient to substantially block thetube 55 so that there is no rectied current flow through the volumecontrol resistor 59.

Likewise, if an incoming signal fades below a predetermined level, of ifa transmitter station is tuned in, which has low signal strength at theantenna in use, the voltage drop in the detector resistor 4I will be soreduced that the control grid 59 will reach a negative value such thatcurrent ow through the volume control resistor 69 will stop.

Referring now to the gas tube relay 51, its control-electrode 15 ispreferably maintained at a constant positive potential with respect toits cathode 11 by means of the voltage divider 5I. This biasingpotential is for the purpose of controlling the sensitivity of the relay51 and is applied to the grid 15 through a circuit which may be tracedfrom the cathode 11, through a conductor 56 to a variable tap 10 onvoltage divider 5I, through the voltage divider to ground, throughground to the volume control resistor 69, and through the volume controlresistor and the conductor 68 to the control grid 15.

So long as there is an incoming signal of suiiicient strength, thecurrent flow in the resistor 69 maintains the grid 15 suiiicientlynegative to prevent the relay tube 51 from operating. As soon as asufficient reduction in signal strength occurs at the antenna in use,the current flow in resistor 69 is so reduced in value that the negativepotential on grid 15 is reduced to a point where the relay operates.When the relay operates the switch 3 is advanced one step to connect asuccessive antenna with the receiver, as will hereinafter be described.

It will be apparent that, by adjusting the variable tap 1U, the signallevel at which the system ,F c v 2,059,081

operates to switch in a different antenna may be determined. Such anadjustment is desirable since the ratio of signal strength to noiselevel will vary from day to day. If this ratio is high, the signalstrength at the antenna in use may drop to a comparatively low levelbefore there isv any object in having a different antenna switched in.On the other hand, if the noise level is high, it may be desirable tohave a different antenna switched in in response to a small drop insignal strength.

Operating voltage for the tube relay 51 is supplied through anothersecondary winding 19 of the transformer 61.

An electromagnet or solenoid 8| connected in series with the platecircuit of the tube relay 51 actuates the pawl 2| for stepping aroundthe ratchet 23. The relay plate circuit may be traced from the plate 83of the relay tube 51 through the secondary winding 19 of the supplytransformer 61, through the electromagnet 8 I, through a spring contactarm 85 on the pawl 2|, and through a contact point 81 to the cathode 11of the relay tube.

A condenser 89 is connected across the contact points and theelectromagnet 8| in series to prevent any alternating current fromflowing through the latter.

In operation, as soon as the potential of the control grid 15 is reducedsufficiently due to interference patterns, fading, or the like, at thefirst antenna, a gaseous discharge takes place in the relay tube andcurrent flows through the solenoid 8| to pull the pawl 2| downwardly androtate the ratchet 23 one step, thereby connecting a second antenna tothe receiver. the downward movement of the pawl, the relay plate circuitis broken at the contact point 81 and the discharge in the relay tube 51is stopped. When the pawl returns to its original position, the actionwill be repeated to connect the third antenna to the receiver inputcircuit if the signal strength at the second antenna is not sufilicientto return the grid 'I5 of the relay tube to its normal highly negativecondition. In other Words, the switch 3 is stepped around continuouslyto switch the antennas successively to the receiver input circuit untilan antenna is finally switched in which is receiving signal energy atthe desired level.

From the foregoing description it will be seen, also, that, when tuningin a transmitting station, the antenna switch 3 is stepped around duringthe period that no signal is tuned in. The switch mechanism is designedto operate rapidly so that if the antenna which is connected to thereceiver at the instant the receiver is tuned in to a certain signal isat a point of low signal strength, a different antenna will be switchedin before the receiver is tuned off the said signal.

The circuit described above is designedto overcome the detrimentaleffects due to interference patterns or due to fading caused by avariation in the amplitude of the incoming carrier wave and side bands..Selective fading, hereinbefore referred to, often occurs due to areduction in the amplitude of the carrier with respect to the amplitudeof the side bands, that is, the carrier may drop to zero or almost zeroamplitude, while the side bands remain at their normal amplitude. Or,the amplitude of the side bands may increase while the amplitude of thecarrier remains unchanged. In order to switch the receiver to adifferent antenna when this type of fading ocn At the end of Y VUMI ULIcurs, an additional relay circuit may be provided, as shown in Fig. 1.

'Ihis additional circuit comprises a carrier wave amplifier 9| and acarrier wave and side band amplifier 93 which have their input circuitsconnected in parallel and coupled to the output circuits of the firstdetector 21 through a transformer winding 95.

The carrier wave amplifier 9| is provided with an output circuit whichis coupled to a diode rectifier 91 through a tuned intermediate-frequenoy transformer 99. The circuit of the rectifier 91 includes aresistor IOI connected in series with the secondary of the transformer99 and shunted by an intermediate frequency bypass condenser |03.

In a similar mannen-the carrier wave and side band amplifier 93 isprovided with an output circuit which is coupled to a second dioderectifier |05 through an intermediate frequency trans former |01. Thecircuit of this rectifier also includes a resistor |09 shunted by anintermediate frequency bypass condenser III.

The output energy of the diode rectifiers 91 and |05 control a gas tuberelay I|3 which operates a second pawl I|5 for stepping around theratchet 23.

The tube relay ||3 is provided with a control electrode |I1 which isconnected to that end of lthe resistor IOI which becomes negative when asignal is received. The cathodes of the rectifiers 91 and |05 areconnected together through a conductor II9, and that end of the resistor|09 which becomes negative is connected to the negative end of a voltagedivider I2 I The voltage divider I2I may be supplied with direct currentfrom any suitable sourcesuch as the'power supply unit 53 in the samemanner as the divider 5|, leads thereto being omitted toy simplify thecircuit diagram.

The input circuit of the relay tube II3 is completed by a connectionfrom its cathode |23 to a point of positive potential on the voltagedivider |2I. This input circuit may now be traced from the controlelectrode II 1, through the resistor I 0I and the conductor II9, throughthe second resistor |09 to the voltage divider I2 I and throughthevoltage divider to the cathode |23.

It will be noted that the resistors I0| and |09 are connected inopposition so that if there is the same voltage drop in each resistor,as will hereinafter be pointed out, the control electrode I1 will bemaintained at a negative potential with respect to its cathode |23. Thisnegative potential is made high enough normally to prevent the relay II3 from operating.

The plate circuit of the relay II3 is supplied with alternating currentthrough a transformer I 25. When the relay II3 is caused to operate,this current is rectified by the relay tube I I3 and is supplied to asolenoid^|21 which operates the pawl II5. `As in the previouslydescribed relay circuit, the pawl IIS is provided with a spring switcharm |29 which is in contact with a contact point I3I until the pawl I|5is pulled down to its bottom position, at which time the plate circuitis broken at the contact point |3I to stop the discharge through therelay tube I I3.

A lter condenser |33 is connected across the solenoid |21 and thecontact points for preventing alternating current pulsations frompassing through the solenoid winding.

If the carrier wave and its side bands are at the proper relativeamplitude, the voltage drop in the resistors and |09 will be equal andthe control grid ||1 will be sufficiently negative to prevent platecurrent flowing in the relay tube ||3.

If, however, selective fading occurs, the voltage drop in resistor ||l|will become less than the voltage drop in the resistor |09 and the grid||1 becomes less negative. If the negative potential on the grid ||1 isreduced below a certain value, an electric discharge takes place in thetube ||3, and the resulting flow of current through the solenoid |21pulls the pawl ||5 down to step the switch arm I3 around to the nextantenna position. This action will continue until an antenna, located ata point where there is no selective fading, is connected to the receiverinput circuit.

While my above-described circuit may be arranged to operate in responseto selective fading of any degree or appreciable range, it is preferablydesigned to respond to selective fading of sumcient magnitude to makethe modulation of the incoming signal exceed This is for the reason thatunless selective fading is of this magnitude, it does not ordinarilycause serious distortion of the signal.

Obviously, the selective fading control circuit and the amplitude fadingcontrol circuit may be used together, as illustrated, or either one maybe used separately. 1f they are used together, either type of fadingwill cause a different antenna to be switched automatically to thereceiver input circuit.

Since the carrier wave amplifier 9| should be tuned very sharply, it maybe desirable to utilize a quartz crystal filter. Instead of using aseparate carrier and side band amplifier 93, as illustrated, theintermediate frequency amplifier 33 in the signal channel may be used.In that case, the amplifier 93 is omitted, and theinput circuit of thediode rectifier |05 is coupled to the output circuit of the intermediatefrequency amplifier 33 in any suitable manner, such a's the similarcircuit for the rectifier 33.

The gas tube relays referred to above may be of any suitable type suchas an electric discharge device known commercially as a thyratron, or agrid glow tube relay.

A circuit involving the feature of two rectifier tubes fed from separatecircuits and connected in opposition for controlling an electricdischarge device is disclosed and claimed in my co-pending application,Serial No. 712,526, filed February 23, 1934 and assigned to the sameassignee as this application.

It may be preferred to employ coupling tubes between the antennas andthe input circuit of the receiver, as shown in Fig. 2. By employingcoupling tubes it is possible to prevent the occurrence, under certainoperating conditions, of clicks in the receiver at the instant that adifferent antenna ls switched into the circuit.

Referring to Fig. 2, where parts corresponding to those in Fig. 1 areindicated by like reference numerals, the antennas 1, 9, and areprovided with coupling tubes |35, |31, and |39, respectively, eachhaving an output circuit coupled to the input circuit of the receiverthrough a transformer |4| which replaces transformer 5 in Fig. 1.

The antenna 1 is coupled to the input circuit of the coupling tube |35through a coupling resistor |43 and a coupling condenser |45. Thecontrol grid |41 of tube |35 is normally maintained at a high negativepotential which biases the tube to cut-ofi by means of the portion |52of a voltage divider |53, the grid |41 being connected to the negativeterminal of voltage divider |53 through grid leak resistors |49 and |5|connected in series. The tube |35 is also provided with a self-biasingresistor |55 connected between cathode |51 and ground.

A filter condenser |59 is connected between the cathode |51 and thejunction point of the two grid leak resistors- |49 and |5| to aid inpreventing clicks when switching from one antenna to another. This pointon the grid leak resistors is connected to a segment |62 of the antennaswitch 3 through a conductor ISI, an opposite segment |64 of the switchbeing connected to the lower end of the cathode biasing resistor |55through a conductor |63.

If theswitch arm I3 is not in contact with the switch segment |62, thecoupling tube |35 is blocked so that the antenna 1 is, in effect,disconnectedfrom the inputjmauit of the receiver. If the switch arm |3is rotated to connect; the switch segment |62 with the opposite switchsegment |64, the lower end of the grid leak resistor |49 is connecteddirectly to the lower end of the cathode biasing resistor |55, wherebythe negative potential on the control grid |41 is reduced to a pointwhere the tube |35 becomes effective to transfer energy from the antenna1 to the receiver.

The time constants of the circuit of condenser |59 and of the circuit ofcondenser |45 are made such that the tube |35 gradually becomesoperative when switch arm |3 makes contact with segment |62, andgradually becomes inoperative when switch arm I3 is moved out of contactwith segment |62. The condenser circuits of the coupling tubes |31 and|39 are similarly adjusted. In operation, therefore, one coupling tubeis gradually becoming operative as another coupling tube is becominginoperativeL The time constants necessary to prevent clicks in thereceiver will not be the same under all conditions. For example, if thereceiver has a very low frequency response, the time constants should begreater than when utilizing a receiver which does not respond to lowfrequency impulses.

It may be noted that if the coupling condenser |45 is given a very smallvalue, the time constant of its circuit may be ignored and the tubecircuit adjusted to avoid clicks solely by the adjustment of the circuitof condenser |59.

The tube coupling circuits for the antennas 9 and are identical with theone described above, the lower ends of the cathode biasing resistors|55, |61, and |69 being connected together by the conductor |63. Also,the lower ends of the grid leak resistors |5|, |1|, and |13 are allconnected to the negative terminal of the voltage divider |53 through aconductor |15 so that each coupling tube is blocked until its gridpotential is reduced by the operation of the antenna switch 3.

Instead of utilizing a plurality of antennas spaced apartgeographically, a plurality of different types of antennas located atapproximately the same point may be employed since a signal may havedifferent fading characteristics when received by different types ofantennas. For example, a loop antenna and a vertical non-directiveantenna may be arranged to be connected successively to the receiver inthe event fading occurs. Since various forms of antennas, including theabove. are well known, and since any type su "o limit# may be used, thesame are not shown and are represented by the antennas l, 9 and Il.

It will be understood that the cathodes of the electric discharge tubesshown in Figs. l and 2 may be either directly or indirectly heated bycurrent supplied from any suitable source (not shown).

Various other modifications may be made in myinvention without departingfrom the spirit and scope thereof and I desire, therefore, that onlysuch limitations shall be placed thereon as are necessitated by theprior art and set .forth in the appended claims.

I claim as my invention:

1. A radio system comprising ,amiuraiity of non-directive,geographicallygadantennas, a radio receiver having a common inputcircuit for si'dtennas and an output circuit, means forselectivelywconnec r singly to sllnputcircuit, and means responsive tothe characteristic of a signal received by said receiver for controllingsaid first means.

2. A radio system comprising a plurality of antennas, an amplifiertunable to an incoming signal, said amplifier having a common inputcircuit for said antennas and an output circuit, and

means for selectively connecting said antennas` ksingly to said inputcircuit in response to fading of the signal energy in said outputcircuit.

3. In combination, a plurality of antennas, a superheterodyne receiverhaving an input circuit, said receiver including means for converting anincoming signal to an intermediate frequency signal, and means includinga circuit tuned to said intermediate frequency for selectivelyconnecting said antennas singly to said input circuit in response tofading of an incoming signal.

4. In combination, a plurality of antennas, a tunable selecting circuithaving an input circuit, means for successively connecting said antennassingly to said input circuit. and means for actuating said rst means inresponse to the fading of an incoming signal.

5. In combination, a plurality of antennas, a radio frequency amplifierhaving an input circuit for said antennas and an output circuit, agastube relay having a control electrode,means including a resistorconnected to said output circuit and with said control electrode formaintaining said control electrode negative so long as the incomingsignal at said input circuit is above a certain strength, and meansincluding said gas tube relay for selectively connecting in rotation asingle and different one of said antennas to said input circuit-inresponse to fading of said signal.

6. In combination, a plurality of antennas, an amplifier having an inputcircuit andan outputv circuit, switching means for successivelyconnecting said antennas to Ysaid input circuit, means including a gastube relay for actuating said switching means, said tube relay includinga control electrode, means for maintaining said control electrodenegative so long as the incoming signal does not fade, means forlowering the negative potential on said control electrode in response tofading of said incoming signal whereby an electric discharge takes placein said tube relay, and means for stopping said discharge in response tothe actuation of said switching means.

7. In combination, a plurality of antennas, a superheterodyne receiverhaving an input circuit, said receiver including means for converting anincoming signal to an intermediate frequency signal, switching means forsuccessively GBI'CH HOO] connecting said antennas to said input circuit,means including a gas tube relay for actuating said switching means,saidtube relay including a control electrode, means including a circuittuned to said intermediate frequency normally ior maintaining saidcontrol electrode negative, means for lowering the negative potential onsaid control electrode in response to fading of an in coming signalwhereby an electric discharge takes place in said tube relay, and meansfor stopping said discharge in response to the actuation of saidswitching means.

8. In combination, a plurality of antennas for receiving an incomingsignal, an amplifier having an input circuit and an output circuit,switching means for successi onnecting said antennas to said inputcircuit, an electric discharge tube having a control grid, meansincluding a rectifier coupled to said output circuit for biasing saidelectric discharge tube substantially to cut-olf when said incomingsignal is below a predetermined strength, a gas tube relay having acontrol electrode, means including said electric discharge tube forapplying a negative bias to. said control electrode, means for reducingsaid negative bias in response to the biasing of said electric dischargetube substantially to cut-off whereby said relay is operated, and meansfor actuating said switching means in response to the operation of saidrelay.

9. A radio system for supplying signals to the input circuit of anamplifier tunable to an incoming signal, said system comprising aplurality of antennas, a plurality of coupling tubes having inputcircuits and output circuits, each of said antennas being coupled tosaid 'amplifier input circuit through one of said coupling tubes, meansincluding said coupling tubes for selectively connecting said antennasto said input circuit in accordance with the signal energy in saidoutput circuit, and means including filters in said coupling tube inputcircuits for preventing a change in antenna .connection from causing asudden change in energy in said amplier output circuit.

10. Electrical apparatus comprising means for receivingaginodulatedradio wave, an lelectric discharge tube for"V controlling saidapparatus, said tube having' a control electrode, bias means forrendering said tube substantially inoperative, two rectifiers havingoutput circuits connected in opposition and means including saidrectifiers for rendering said tube operative in response to apredetermined change in percentage modulation of a received carrierwave.

11. Electrical apparatus comprising means for receiving a modulatedradio wave, a translating device, an electric discharge tube having acontrol electrode, said tube having an output circuit coupled to saidtranslating device, two rectiiiers having output circuits connected inopposition, means for connecting said control electrode to said outputcircuits, and means including said rectiers for controlling thepotential on said control electrode in response to a predeterminedchange in percentage modulation of said radio wave.

l2. In combination, a plurality of antennas, radio receiving apparatushaving an input circuit, means for successively connecting said antennasto said input circuit, and potential controllable means for actuatingsaid first means in response to selective fading of an incomingmodulated carrier wave, said last named means including a controllingsource oi potential variable in response to changes in the relativestrength of the carrier wave and the side bands thereof, and anactuating device connected therewith and responsive to said potentialvariations.

13. In a diversity radio receiving system, antenna selecting meanshaving a movable selecting element, means providing a signal amplifyingchannel connected with said selecting means to receive and amplify asignal from a selected antenna, means associated with said signalchannel for receiving a signal and separating the carrier wave withside-bands and the carrier-wave thereof to provide opposing controlpotentials responsive to changes in the relative amplitude of saidside-bands and carrier wave, and means responsive to a predetermineddifference in said opposing potentials for actuating said selectingelement.

14. In a diversity radio receiving system, antenna selecting meanshaving a movable selecting element, step-by-step motor means for movingsaid selecting element, means providing a signal amplifying channelconnected with said selecting means to receive and amplify a signal froma selected antenna, tuning means in said channel for causing said systemto be responsive to signals within a predetermined frequency band, meansassociated with said signal channel for receiving a signal and forseparating the carrier-wave with side-bands and the carrierwave thereofto provide opposing control potentials responsive to changes in therelative amplitude of said side-bands and carrier-wave, and meansresponsive to a predetermined difference in said opposing potentials foractuating said motor means successively during the existence of saidpotential difference, said last named means including an actuatingelectro-magnet for said motor means, a grid-controlled electricdischarge device in circuit with said electro-magnet for controlling theflow of operating current thereto, means for applying said potentialdifference to said device to cause current flow therethrough to energizesaid electro-magnet, and means for stopping said current flow inresponse to the operation of said step-by-step motor.

15. In a diversity radio receiving system, the combination of a singleradio receiver, means providing a plurality of signal supply sources,means having a movable selecting element for successively connectingsaid receiver with said signal supply sources, means for moving saidselecting means in response to a predetermined low average signalamplitude, means for separating the carrier-wave with side-bands andcarrier Wave of a signal to provide opposing control potentialsresponsive to changes in the relative amplitude of said side-bands andcarrier wave, and means responsive to a predetermined difference in saidpotentials for actuating said selecting means.

16. A radio system comprising a plurality of antennas, a radio receiverhaving an input circuit and an output circuit, means for selectivelyconnecting only one of said antennas at a time to said input circuit andfor disconnecting said previously connected antenna from said receiver,and means responsive to a signal characteristic for controlling saidfirst means.

17. A radio system comprising a plurality of antennas, a radio receiverhaving an input circuit and an output circuit, a switch for selectivelyconnecting said antennas to said input circuit, and means forcontrolling said switch in response to the signal impressed upon saidinput circuit decreasing below a predetermined amplitude.

18. A radio system comprising a plurality of antennas, a radio receiverhaving an input circuit and an output circuit, a switch for selectivelyconnecting said antennas to said input circuit, and means for switchingto a different antenna in response to the signal to which said receiveris tuned and which is impressed upon said input circuit being below apredetermined amplitude.

19. A radio system comprising a plurality of antennas, a radio receiverhaving an input circuit and an output circuit from which output energyis supplied, a switch for selectively connecting said antennas to saidinput circuit, and means for operating said switch in response to sdoutput energy falling below a predetermined v ue.

20. A radio receiving system comprising a plurality of antennas, a radioreceiver having an input circuit, said receiver being tunable to receivea certain modulated carrier wave, means for selectively connecting saidantennas to said input circuit, and means responsive to variations inthe relative strength of the carrier wave to the side band strength forcontinuously operating said iirst means until there is connected to saidinput circuit an antenna which has impressed thereon a signal to whichsaid receiver is tuned which has a certain predetermined normal relationbetween the strength of the carrier wave and the side-bands thereof.

21. A radio receiving system comprising a plurality of antennas, a.radio receiver having an input circuit, said receiver being`tiinable toreceive a certain modulated carrier wave, means for selectivelyconnecting said antennas to said input circuit, potential controllablemeans for continuously operating said first means, and means connectedwith said last named means and responsive to a received modulatedcarrier Wave for applying a controlling potential to said last namedmeans, and means for causing said potential to change in response tochange in carrier wave strength with respect to the strength of theside-bands thereof.

22. A radio system comprising a plurality of antennas, a radio receiverhaving an input circuit and including means for demodulating a modulatedcarrier wave to reproduce the modulating signal, means for selectivelyconnecting said antennas to said input circuit, and means responsive tofading of said modulated carrier Wave for controlling said antennaselecting means.

23. The combination with a radio receiver having algmnawrngiontrolmeans, of a plurality of signal energy collectors; a signal inputcircuit for said receiver, switch means for selectively connecting saidenergy collectors successively to said circuit, and means connected withand responsive to said automatic volume control means for actuating saidswitch means.

24. In a radio signal receiving system for modulated carrier wavesignals, the combination of means providing a plurality of signalsources, means providing a signal amplifying channel including an inputcircuit, means for selectively connecting said input circuit with eachof said signal sources successively, means for actuating said last namedmeans responsive to variations in a controlling potential, a secondmeans for actuating said selective collector means in respouse tovariations in a controlling potential,

4"; means for supplying a control potential to said A rst namedactuating means responsive to variations'in the strength of a receivedcarrier wave,

,and means for applying a controlling potential to the second actuatingmeans in response to variations in the strength of the received carrierwave with respect to the strength of its sidebands.

25. A radio system comprising a plurality of geographically spacedantennas.'a radio receiver having a common input circuit for saidantennas and an output circuit, means for selectively connecting saidantennas singly to said input cir-

